Secret British tests on an exploded bomb are thought to prove Syrian troops have launched at least one chemical attack on rebels there.
Samples from the army mortar bomb were smuggled out of the warzone by agents working for MI6.
Spies then flew the cargo back to the UK, where it tested positive for deadly nerve agent Sarin.
A senior Western source said last night: “The British believe they have conclusive evidence, but this is a very delicate situation.”
It comes after photos emerged of one victim foaming from the mouth in hospital after an apparent chemical attack and reports people suffered breathing problems and vomiting in a bombing last month.
Around 92,000 people have been killed in the two-year civil war so far.
The latest results, taken five weeks ago at the military’s Porton Down research centre, in Wiltshire, have been passed to the British government, the US and France.
But American top brass want further proof Sarin has been used by President Bashar al-Assad’s troops since the start of the uprising against him, such as samples from a victim’s hair and clothes, or the soil.
The source said: “MI6 played the leading role but the American military wants more evidence before it agrees Assad has crossed the line in the use of chemical weapons.
“The question is what is the West going to do now? If nobody reacts, there was not much point in conducting the tests.
“America, Britain and France are desperately trying to avoid being sucked into a proxy conflict with Syria’s supporter Iran, which is why politicians are being coy.”
Experts in the UK fear Iran put pressure on Assad’s forces to fire the weapons to test the West’s immediate reaction.
And yesterday the BBC claimed it had further evidence of a chemical attack in Syria last month.
Witnesses in the northern town of Saraqeb said helicopters dropped at least two devices containing poisonous gas.
Doctors treated eight people with breathing problems. Some were vomiting and others had constricted pupils.
The CIA believes Syria has had a chemical weapons programme “for years and has a stockpile which can be delivered by aircraft, ballistic missile and artillery rockets”.
The chemical scare came as footage yesterday emerged of a group of Assad’s supporters allegedly being shot dead by the jihadist al-Nusra Front group.
What is Sarin?
Sarin was first produced by German scientists in 1938, although it was never used in the Second World War.
It is a colourless, odourless gas and a dose of just 0.5mg can kill in minutes.
It is either absorbed through the skin or inhaled. Victims are paralysed and then die of asphyxiation.
Sarin is a member of the organophosphate family of biochemicals, which were once used as pesticides before their effects were fully known.